The present invention relates to a method and apparatus for recovering heat energy and kinetic energy possessed by a gas discharged from the top of a blast furnace by supplying this exhaust gas to a turbine and converting the heat energy and kinetic energy to an electric energy by a generator driven by the turbine.
A gas discharged from the top of a blast furnace has large quantities of heat energy and kinetic energy, and it is desired to recover such energies effectively without wasteful discharge and use them as the power for other purposes.
According to the conventional method and apparatus for recovering the energy of this exhaust gas, a gas discharged from a blast furnace is passed through a dust precipitator to remove dust therefrom, the cleaned exhaust gas is branched into a septum valve and a turbine, and the energy is recovered as electric energy by driving a generator driven by the turbine while the top pressure of the blast furnace is being controlled by the septum valve. In this conventional technique the flow rates of the exhaust gas supplied to the septum valve and into the turbine are set according to two methods. Even during the normal steady operation of the blast furnace, the flow rate of the exhaust gas discharged from the top of the blast furnace gas varies with the lapse of time. According to the first method, the capacity of the turbine is set at the minimum value of the bottom which is not influenced by the variation of the total flow rate of the exhaust gas, namely at a level indicated by line A--A of FIG. 1. An excess portion of the exhaust gas exceeding the capacity of the turbine is supplied to the septum valve and the flow rate is controlled by the septum valve so as to maintain a necessary top pressure. According to the second method, the flow rate of the exhaust gas supplied to the turbine, namely the capacity of the turbine, is set at a maximum value among varying values of the total flow rates of the exhaust gas, namely at a level indicated by line B--B in FIG. 2. The top pressure is controlled by governor valve controlling the flow rate of the exhaust gas supplied to the turbine, and the septum valve is disposed merely to cope with blow-by or is used only when the turbine is stopped.
In the first method, the system can be controlled relatively easily, but since the quantity of the exhaust gas flowing in the septum valve is large, the quantity of the exhaust gas discharged without recovery of energy is increased. Therefore, the ratio of energy recovered by the turbine is low.
In the second method, when the flow rate of the exhaust gas varies and the flow rate of the gas flowing into the turbine is lower than the capacity of the turbine, in order to maintain the top pressure at a desirable level, it is necessary to throttle the flow of the gas. In this case, the loss of the gas by throttling is large and the energy recovery ratio is rather reduced. Especially when the blast furnace should be operated at a low operation rate over a period of a long time, this reduction of the energy recovery ratio is conspicuous. Moreover, since the planned capacity of the turbine becomes larger, the dimension of the turbine should be increased and hence, also dimensions of accessory equipments should inevitably be increased, resulting in increase of the equipment cost.